Electronic ballast

ABSTRACT

The invention relates to a ballast for low-pressure discharge lamps, comprising a controllable inverter circuit for the generation of a high-frequency supply voltage for a discharge lamp ( 1 ), a lamp inductance (L 1 ) connected to the inverter circuit, a lamp parallel capacitor (C 1 ) which is serially connected to the lamp inductance (L 1 ) and connected in parallel to the discharge lamp ( 1 ), and a preheating circuit supplying heating current to the electrodes ( 2, 3 ) of the discharge lamp ( 1 ). In order to minimize electrical losses and to enable universal use of said ballast for various types of discharge lamps the invention proposes that current is supplied to the preheating circuit via an auxiliary winding ( 7 ) arranged on the lamp inductance (L 1 ) with said auxiliary winding ( 7 ) being connected with the preheating circuit via a controllable switch or a parallel resonant circuit ( 9 ).

The invention relates to a ballast for low-pressure discharge lamps, comprising a controllable inverter circuit for the generation of a high-frequency supply voltage for a discharge lamp, a lamp inductance connected to the inverter circuit, a lamp parallel capacitor which is serially connected to the lamp inductance and connected in parallel to the discharge lamp, and a preheating circuit supplying heating current to the electrodes of the discharge lamp.

Such a ballast is known, for example, from what has been disclosed in DE 199 20 030 A1. Using the preheating circuit the electrodes of the discharge lamp designed in the form of heating coils are preheated so that a thermionic emission is initiated. For the ignition of the discharge lamp the frequency of the supply voltage is varied from a preheating frequency to an operating frequency with the help of the controllable inverter circuit. This will cause resonance in the series resonance circuit formed by the lamp inductance and the lamp parallel capacitor so that an ignition voltage is applied to the discharge lamp which is sufficient for ignition purposes.

The prior-art ballast is equipped with a heating transformer the primary winding of which is connected in series,with the lamp parallel capacitor. The secondary windings of the heating transformer supply heating current to the electrodes of the discharge lamp said electrodes being designed in the form of heating coils. The preheating voltage applied to the primary winding of the heating transformer is thus exclusively governed by the voltage drop across the lamp parallel capacitor. During the transient period when the preheating frequency changes until the operating frequency is reached this voltage increases causing the heating current to go up as well. However, the heating current is limited as a result of the saturation occurring in the heating transformer. Upon ignition of the discharge lamp the voltage impressed on the lamp parallel capacitor collapses and drops to the operating voltage level of the lamp. The heating current flowing through the heating coils of the electrodes when the lamp's operating state is reached decreases accordingly.

A drawback with such prior-art ballast is, however, that even during the ongoing operation of the discharge lamp a heating current is constantly applied which leads to higher power consumption. This is due to the fact that during lamp operation the primary winding of the heating transformer is constantly supplied with reactive current flowing through the lamp parallel capacitor. Another disadvantage to be associated with the prior-art ballast is experienced when the discharge lamp is switched on causing an undesirably high heating current to flow through the cold heating coils of the discharge lamp's electrodes. The heating current is only be limited by the saturation of the heating transformer. In the event the prior-art ballast is used with discharge lamps having particularly sensitive electrodes damage of the heating coils may occur.

Proceeding from these considerations it is the object of the present invention to provide an electronic ballast for low-pressure discharge lamps that is designed such that losses caused by the preheating circuit are minimized and which can be universally used, in particular also for discharge lamps equipped with sensitive electrodes.

Based on a ballast of the kind described above this objective is reached in such a manner that current is supplied to the preheating circuit via an auxiliary winding arranged on the lamp inductance with said auxiliary winding being connected with the preheating circuit via a controllable switch or a parallel resonant circuit.

Due to the fact that, according to the invention, current is supplied to the preheating circuit of the ballast via an auxiliary winding arranged on the lamp inductance the power supply of the preheating circuit—other than with the prior-art ballast—is not dependant on the reactive current flowing through the lamp parallel capacitor. Moreover, by making use of an auxiliary winding located on the lamp inductance another advantage is gained in that the heating current arising during the cut-in operation can be gradually increased by slowly lowering the supply voltage frequency from a high preheating frequency down to operating frequency. In this way an excessively high and for the electrodes of the discharge lamp harmful heating current is avoided. It is also beneficial that with the ballast according to the invention the power supply to the preheating circuit can be interrupted with the aid of a controllable switch or by means of the parallel resonant circuit while the lamp is operating continuously. This results in effectively eliminating undesirable losses otherwise arising because of the heating current which is constantly flowing during lamp operation. As controllable switch a transistor can be used in a customary manner by means of which the electrical connection between the auxiliary winding arranged on the lamp inductance and the preheating circuit is interrupted.

It is considered advantageous when the preheating circuit of the ballast according to the invention comprises a heating transformer the primary winding of which is connected with the auxiliary winding and whose secondary winding is connected with the electrodes of the discharge lamp. Accordingly, the primary winding is fed by the auxiliary winding arranged on the lamp inductance, with one secondary winding each being provided for heating one electrode of the discharge lamp. The saturation effect which occurs in the heating transformer when a heavy current flows through the lamp inductance can be utilized for the purpose of limiting the heating current. It is viewed expedient for this purpose to provide a heating transformer which is of toroidal design.

Expediently, with the ballast according to the invention the frequency of the supply voltage should be variable by means of the controllable inverter circuit ranging from a preheating frequency for electrode preheating to a different operating frequency used for the continuous operation of the discharge lamp. In a particularly simple wiring manner the power supply to the preheating circuit may then be interrupted in such a manner that the parallel resonant circuit connected between auxiliary winding on the lamp inductance and the preheating circuit is suitably matched to the operating frequency of the discharge lamp.

An embodiment of the ballast according to the invention is explained below by way of the circuit shown in the diagram.

A supply voltage U_(b) is applied to a half-bridge consisting of two solid-state switches T₁ and T₂. The half-bridge circuit consisting of the two solid-state switches T₁ and T₂ forms part of a controllable inverter circuit for the generation of a high-frequency supply voltage for a discharge lamp 1. As necessitated by the desired frequency of the supply voltage the solid-state switches T₁ und T₂ are alternately activated and deactivated by an electronic control circuit not shown in more detail in the diagram. Via lamp inductance L₁ the discharge lamp 1 is connected with the half-bridge consisting of the two switches T₁ and T₂. A lamp parallel capacitor C₁ is connected in parallel with discharge lamp 1 and serially connected to lamp inductance L₁. Furthermore, a preheating circuit is provided via which heating current is supplied to electrodes 2 and 3 of discharge lamp 1. The preheating circuit consists of a toroidal transformer 4 the secondary windings 5 and 6 of which are connected to electrodes 2 and 3. In accordance with the invention current is supplied to the preheating circuit by an auxiliary winding 7 arranged on lamp inductance L₁. Auxiliary winding 7 is connected to a primary winding 8 of the toroidal transformer 4 with a parallel resonant circuit 9 being connected between auxiliary winding 7 and primary winding 8. Parallel resonant circuit 9 is matched to the operating frequency of discharge lamp 1 in such a way that during continuous operation of discharge lamp 1 only a minimum current flows through primary winding 8 of the toroidal transformer 4. According to the embodiment shown in the figure capacitors C_(k), and C_(k2) have been provided and serve for direct-current decoupling. Capacitor C_(h) connected between lamp inductance L₁ and discharge lamp 1 also serves the purpose of direct-current decoupling.

Switching discharge lamp 1 on initially causes a supply voltage to be generated by means of solid-state switches T₁ and T₂ the frequency of which corresponds to that of a preheating frequency. The current flowing at this frequency through lamp inductance L₁ induces a voltage in auxiliary winding 7 that causes a current flow in primary winding 8 of the heating transformer 4. With the preheating frequency prevailing, the parallel resonant circuit 9 is distinctly outside its resonance range. The current flowing through primary winding 8 results in a current flowing through electrodes 2 and 3 which are fed via the secondary windings 5 and 6 of heating transformer 4. Now the frequency of the supply voltage is lowered by means of the controllable inverter circuit from preheating frequency to an operating frequency which is different and used for the continuous operation of discharge lamp 1. When said frequency is lowered the series resonant circuit consisting of lamp inductance L₁ and lamp parallel capacitor C₁ becomes resonant and causes an increasingly higher voltage to be applied to discharge lamp 1. As soon as the ignition voltage is reached discharge lamp 1 ignites and the voltage drops sharply until the operating voltage of discharge lamp 1 is reached. At operating frequency resonance prevails in the parallel resonant circuit 9 so that the heating current flowing through electrodes 2 and 3 is greatly reduced. While lamp 1 is in continuous operation only a minimum heating current thus flows through electrodes 2 and 3. 

1. Ballast for low-pressure discharge lamps, comprising a controllable inverter circuit for the generation of a high-frequency supply voltage for a discharge lamp (1), a lamp inductance (L₁) connected to the inverter circuit, a lamp parallel capacitor (C₁) which is serially connected to the lamp inductance (L₁) and connected in parallel to the discharge lamp (1), and a preheating circuit supplying heating current to the electrodes (2, 3) of the discharge lamp (1), wherein the current is supplied to the preheating circuit via an auxiliary winding (7) arranged on the lamp inductance (L₁) with said auxiliary winding (7) being connected with the preheating circuit via a controllable switch or a parallel resonant circuit (9).
 2. Ballast according to claim 1, wherein the preheating circuit comprises a heating transformer (4), the primary winding (8) of which is connected to auxiliary winding (7) and whose secondary windings (5, 6) are connected to electrodes (2, 3) of the discharge lamp (1).
 3. Ballast according to claim 2, wherein the heating transformer (4) is of toroidal design.
 4. Ballast according to claim 1, wherein the frequency of the supply voltage can be varied by means of the controllable inverter circuit from a preheating frequency for electrode (2, 3) preheating to a different operating frequency used for the continuous operation of the discharge lamp (1).
 5. Ballast according to claim 4, wherein the parallel resonant circuit (9) is matched to the operating frequency in such a way that during continuous operation of the discharge lamp (1) the power supply of the preheating circuit is interrupted. 